Added ability to search for nearby stores

Stores now have a longitude and latitude. The findAllNearLocation
method finds all stores in the stores collection that are within the
specified number of miles from the given longitude and latitude
coordinates.

findAllNearLocation relies on the helper methods
milesToDegreesLongitude and milesToDegreesLatitude which convert miles
to degrees longitude and latitude respectively. The constants in these
methods are estimated values for the continental United States and do
not account for the changing curvature of the earth. It is clear that
they’re fairly inaccurate (e.g., a store that is 13 miles away still
comes up in the search results when asked for stores within a 6 mile
radius).